Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and a blade root of the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
The amount of power that may be produced by a wind turbine is typically constrained by structural limitations of the individual wind turbine components. The power available from the wind is proportional to the area of the rotor, and the square of the rotor diameter. Thus, the amount of power produced at different wind speeds can be significantly higher by increasing the diameter of the rotor of the wind turbine. Such an increase in rotor size, however, also increases mechanical loads and material costs with what may not be a proportional increase in energy production. Further, though it is helpful to control power and rotor speed, thrust from the wind on the rotor truly drives many dominant fatigue loads, along with any asymmetry of that thrust. The terms “thrust,” “thrust value,” “thrust parameter” or similar as used herein are meant to encompass a force acting on the wind turbine due to the wind and in the general direction of the wind. The thrust force comes from a change in pressure as the wind passes the wind turbine and slows down. Further, the terms “thrust,” “thrust value,” “thrust parameter” or similar as used herein may describe an input to a control method, a value that changes in direct proportion to thrust in an operating region of interest (e.g. individual or average out-of-plane blade or flapwise bending, tower bending, or tower top acceleration), or an estimate of thrust based upon any combination of the above quantities or with other standard measured quantities such as wind speed, speed, or power of the machine. The terms “thrust,” “thrust value,” “thrust parameter” or similar may also describe a forward-looking estimate of future thrust, e.g. as determined by a sensor that measures wind speed upwind of the rotor plane.
Recent developments in the wind industry have led to new methods of mechanical-load-reducing controls that allow larger rotor diameters to be employed with less than proportional increases in material costs. For example, some modern wind turbines may implement drive train and tower dampers to reduce loads. In addition, modern wind turbines may utilize individual and collective blade pitch control mechanisms to reduce fatigue and extreme loads, thereby enabling higher ratios between rotor diameter and structural loads while also lowering the cost of energy.
Still additional wind turbines have employed partial control of thrust, such as “peak shavers,” “thrust clippers,” and/or “thrust control” in the peak thrust regions only. Such control technologies may implement limitations on fine pitch settings in certain conditions, or other variants, but do not employ a full closed-loop control on thrust. Though thrust is related to power and speed of the wind turbine, the thrust is not synonymous or linearly proportional with either. Thus, in some operating regions, it may be possible to change the thrust acting on the wind turbine through controls with less than proportional effect on power, or vice versa. Further, it may be possible to control speed and thrust almost independently in some regions, (e.g. when considering dynamic excursions from a mean value rather than long-term average values), however, current control technologies do not control speed and thrust in this manner. In addition, many modern control techniques do not address thrust control and/or even accentuate thrust variations in attempting to maintain constant power output through certain conditions.
Accordingly, a system and method that addresses the aforementioned problems would be welcomed in the technology. For example, a system and method that incorporates thrust-speed control to increase rotor diameter at a given structural mass and/or energy production while also reducing loads acting on the turbine would be advantageous.